There have long been demands for high-luminance, light emitting device capable of causing short-wavelength emission in the blue light region. Such light emitting device has recently been realized by using AlGaInN-base materials. Rapid progress has also been made in applying the device to full-color, light emitting apparatuses or to display apparatuses by combining it with red and green high-luminance, light emitting devices. Use of the AlGaInN-base material, however, inevitably raises the costs because the material contains Ga and In as major components, both of which are relatively rare metals. One of other major problems of the material is that the growth temperature thereof is as high as 700 to 1,000° C., and thus consumes a considerably large amount of energy for the production. This is undesirable not only in terms of cost reduction, but also in terms of being against the stream of the times where discussions on energy saving and suppression of global warming are prevailing. Japanese Laid-Open Patent Publication No. 2001-44500 proposes a light emitting device having a more inexpensive ZnO-base compound semiconductor layer heteroepitaxially grown on a sapphire substrate. Japanese Laid-Open Patent Publication No. 11-168262 discloses a two-dimensional-array planar light emitting device using a light emitting layer portion composed of oxides of Zn and Mg, or alloy thereof.
In addition, an InAlAsP/InGaAsP compound semiconductor laser typically for use in transponders for submarine optical fiber cables, of which specifications such as crystal defect density or the like are very strictly regulated in order to realize a high output and an high durability.
In all of these devices, semiconductor layers composing the light emitting layer portion are formed by a vapor-phase epitaxy process such as sputtering, molecular beam epitaxy (MBE) or metal organic vapor phase epitaxy (MOVPE).
There is a problem that oxide layers of Zn and Mg are very likely to cause oxygen deficiency, and they inevitably tend to have an n-type conductivity, so that it is intrinsically difficult to obtain the crystal having only a less amount of n-type carrier (electrons) as a conductive carrier. Nevertheless, in the fabrication of the electronic devices disclosed in the above-described patent publications, it is essentially necessary to form oxide layers of Zn and Mg having a p-type conductivity. These oxide crystals, however, tend to have an n-type conductivity due to oxygen deficiency as described in the above, and it has long been believed as very difficult to form the p-type crystal or non-doped, semi-insulating crystal used for the active layer. One possible method may be such as adding p-type dopant, but conversion of an n-type conductivity of a material into a p-type conductivity needs a large amount of dopants in order to compensate the whole portion of the existing n-type carriers and to excessively generate p-type carriers, so that problems in stability, reproducibility and uniformity of the electric characteristics remain unsolved.
Even for the case where the light emitting device is to be fabricated by a vapor-phase epitaxy process using any compound semiconductors other than the oxides of Zn and Mg (referred to as ZnO-base oxide or MgZnO-base oxide, hereinafter), only a tiny crystal defect ascribable to variation in reaction efficiency of the source gases may cause failure especially in the aforementioned InAlAsP/InGaAsP compound semiconductor laser, for which a very high level of quality is required, and may considerably lower the production yield.
ZnO-base oxide can be obtained by a vapor-phase epitaxy in a vacuum environment, where heteroepitaxial growth process using a substrate of a different origin, such as sapphire, is unconditionally adopted because of difficulty in bulk single crystal growth. It is therefore necessary to interpose an appropriate buffer layer between the substrate and the light emitting layer portion in order to attain a desirable crystallinity of the light emitting layer portion as described in the above. The aforementioned Japanese Laid-Open Patent Publication No. 2001-44500 discloses a method in which the buffer layer (contact layer) is formed by MBE (Molecular Beam Epitaxy) process or MOVPE (Metalorganic Vapour Phase Epitaxy) process similarly to the light emitting layer to be formed in succession.
The MBE process, however, cannot readily suppress generation of the oxygen deficiency due to its low pressure in the growth atmosphere, so that it is very difficult for the process to form the ZnO-base oxide layer which is indispensable for composing the light emitting device. On the other hand, the MOVPE process can arbitrarily vary partial pressure of oxygen during the growth, and thus can suppress generation of the oxygen elimination or oxygen deficiency by raising the atmospheric pressure to some extent. In the MOVPE process generally proceeded in a continuous manner, even if any accidental irregularity such as deficiency or dislocation of the atoms should occur, the layer growth for the next layer and thereafter continuously proceed while leaving the irregularity unrepaired, so that the process could not always ensure a desirable quality of the buffer layer which governs the crystal quality of the light emitting layer portion, and this has consequently been making it difficult to obtain the device having an excellent light emission efficiency.
The aforementioned ZnO-base oxide will have a larger band gap energy as alloy composition x of MgO (magnesium oxide) to ZnO (zinc oxide) increases. For the case where the ZnO-base semiconductor light emitting device, which comprises an MgZnO-type oxide, is configured based on the double heterostructure, it is therefore a general practice to compose the active layer with ZnO in view of ensuring more effective confinement of carriers injected thereto. The MgZnO-type oxide can be formed by the MOVPE process or MBE process as described in the above, but the formation process thereof is highly causative of oxygen deficiency of the MgZnO-type oxide and can readily result in degradation of crystallinity of the active layer composed of ZnO. This consequently expands total half value width of the emission wavelength range ascribable to the active layer, reduces the emission intensity, and suppresses the emission efficiency for specific wavelength to be desired.
A first subject of the invention is, therefore, to provide a method of fabricating a light emitting device having a ZnO-base oxide layer, capable of growing the p-type oxide layer in a reproducible and stable manner.
A second subject of the invention is to provide a method of fabricating a light emitting device capable of drastically raising reaction efficiency of the source gases when the semiconductor layer composing the light emitting layer portion is formed by a vapor-phase epitaxy process, and of readily realizing semiconductor layers having a conductivity type which have not conventionally been obtainable, and having only a less amount of crystal defects and being high in quality.
A third subject of the invention is to provide a method of fabricating a light emitting device capable of realizing a high-quality, light emitting layer portion composed of a ZnO-base oxide, and to provide also a light emitting device obtainable by the method.
A fourth subject of the invention is to provide a light emitting device using a ZnO-base oxide, of which active layer can be formed with a high quality in an exact manner, and is further to provide a high-performance, blue-color light emitting device at low costs.